A schematic diagram of a prior art protection circuit for a vapor compression air conditioning system 10 is illustrated in FIG. 1. When a vapor compression air conditioner 10 experiences fault conditions, including high pressure, low pressure or frozen refrigerant, it is necessary to shut down the compressor 12 to prevent damage to the compressor 12 and/or other components of the air conditioner.
In order to turn off the compressor 12 in response to these fault conditions, industry has typically employed a lockout mechanism known as a "Lockout Relay" or an impedance relay 14. The lockout circuitry generally includes a relatively low impedance contactor coil 16 that is used to open and close the main normally open contacts 18 of the air conditioner compressor power circuit. A small control circuit transformer is typically used to supply the control voltage 20 to power the lockout circuitry. In one current path, normally closed lockout relay contacts 22 and conventional sensing switches 24, 26 and 28 designed to respond to high pressure, low pressure and frozen refrigerant conditions respectively within the air conditioner 10, are placed in series with the compressor contactor coil 16. In an alternate path, the compressor contactor coil 16 is placed in series with a relatively high impedance lockout relay coil 30. The lockout relay coil 30 opens the lockout relay contacts 22 under the predefined fault conditions. The lockout relay contacts 22 and sensor switches 24, 26 and 28 are positioned parallel to the lockout relay coil 30.
Under normal operating conditions, the current flows through the compressor contactor coil 16, the closed relay contacts 22 and the closed sensor switches 24, 26 and 28. Sufficient current flows through the compressor contactor coil 16 to maintain the compressor contacts 18 in a closed state and the compressor 12 remains on. In the alternate path, since the lockout relay coil 30 has a relatively high impedance, only minimal current flows through the lockout relay coil 30 and the lockout relay contacts 22 remain closed.
In the event that one of the sensing switches 24, 26 or 28 opens in response to one of the predefined fault conditions within the air conditioner 10, current can only flow through the lockout relay coil 30 and the compressor contactor coil 16. As the impedance of the contactor coil 16 is relatively low, the lockout relay coil 30 now receives sufficient power to open the normally closed lockout relay contacts 22. At the same time, because of the relatively high impedance of the lockout relay coil 30, the compressor contactor coil 16 no longer receives sufficient power to maintain the compressor contacts 18 in a closed state resulting in the compressor 12 being shut down. Even if the open sensing switch 24, 26 or 28 subsequently closes, the lockout relay contacts 22 remain open so that the current continues to flow through the high impedance lockout relay coil 30. The lockout relay coil 30 continues to hold the lockout relay contacts 22 open until the system is reset manually by the opening and then closing of a power supply switch 32.
There are a number of factors that may affect the reliability of the lockout relay circuit. The impedances of the compressor contactor coil 16 and the lockout relay coil 30 must be carefully matched to ensure that when a switch 24, 26 or 28 senses a predefined fault condition, the lockout relay coil 30 has sufficient power to open the lockout relay contacts 22, and at the same time, the compressor contactor coil 16 is deprived of sufficient power to close the compressor contacts 18. Significant impedance variations due to tolerances on the lockout relay coil 30 and the compressor contactor coil 16 components make impedance balancing difficult. Also, since contactors are conventionally used as on/off devices with either a rated voltage or zero voltage across the associated coil, manufacturers do not generally publish sufficient coil operating information, making special inquiries or additional testing of the coils necessary to ascertain the voltage drop values that will assure system operation.
In addition, if the control voltage 20 is not maintained within a narrow operating range, the lockout circuitry may also malfunction. Small circuit control transformers with high secondary voltages often have poor regulation. If the control voltage 20 is too high, there will be sufficient current available to pull in both the lockout relay coil 30 and the compressor contactor coil 16. This will cause the lockout relay coil 30 to open the relay lockout contacts 22, but it will also enable the compressor contactor coil 16 to close the contacts 18 so that the compressor 12 remains on. If the control voltage 20 is too low, there will be insufficient current for the lockout relay coil 30 to maintain the contacts 22 in an open position and the lockout circuit will have the possibility of resetting itself, even if the fault conditions within the air conditioner 10 have not been corrected.
Clearly, it would be desirable to use a lockout mechanism that would eliminate the need for coil impedance balancing, that would not be as susceptible to control voltage fluctuations, and that would ensure that the power to the compressor would remained "locked out" until the fault conditions which initially triggered the lockout circuit had been corrected and the system reset manually. It would also be desirable to use a contactor with a built in lockout mechanism in order to reduce the number of components mounted and the amount of interwiring between components. The present invention seeks to achieve these objectives.